Hantaviruses infect endothelial cells (ECs) and cause 2 vascular permeability-based diseases: Hemorrhagic Fever with Renal Syndrome (HFRS) and Hantavirus Pulmonary Syndrome (HPS). Pathogenic hantaviruses bind to a 53 residue PSI domain present at the apex of bent, inactive, 1v23 integrins and dysregulate 1v23 integrin function (33, 73). The absence of 1v23 function is a known cause of vascular permeability and hemorrhagic disease, and only pathogenic hantaviruses bind 1v23. Hantavirus binding to 23 is RGD independent, however hantavirus attachment proteins and domains required for binding have not been discovered. We have identified 2 differences between surface proteins of pathogenic and non-pathogenic hantaviruses which correlate with 1v23 integrin usage. Here we will investigate the domains and residues of hantavirus surface glycoproteins required for pathogenic hantavirus binding to the 23 integrin PSI domain. Hantavirus surface proteins are synthesized as a polyprotein that is co-translationally cleaved into Gn and Gc fragments and trafficked to the cis-Golgi where hantavirus budding occurs. To date all pathogenic hantaviruses tested use 1v23 integrins for viral entry while non-pathogenic hantaviruses use discrete 1521 integrins. These findings suggest that unique changes in hantavirus Gn or Gc surface proteins are likely to differentiate viral attachment of pathogenic and non-pathogenic hantaviruses. We identified 2 Gc residue changes that are unique to non-pathogenic hantaviruses and not present in highly divergent pathogenic hantaviruses. Our studies of pathogenic hantavirus binding to 23 integrin PSI domains provide a basis for defining hantavirus proteins, domains and residues that mediate attachment. Proposed studies are likely to identify virus specific targets for the development of hantavirus therapeutics and vaccines and antibodies to viral attachment domains are likely to have direct therapeutic application. Objective: In this proposal we will define the domains and residues of pathogenic hantaviruses required for binding to 23 intergrin PSI domains and determine whether these domains elicit neutralizing antibody responses. Specific Aims: 1) Analyze PSI Domain Binding to Pathogenic Hantavirus Surface Proteins 2) GnGc Pseudotyped Virus Will be used to Define Requirements for 1v23 Binding 3) Antibodies to GnGc Peptides Required for PSI Binding will be evaluated for their ability to Inhibit Hantavirus Infection and PSI Domain Binding PUBLIC HEALTH RELEVANCE: Pathogenic hantaviruses bind to a small protein domain on 23 receptors called the PSI domain and targeting the viral protein that is responsible for binding to the 23 receptor is a viable means of inhibiting hantavirus infectivity and may be an antiviral therapeutic approach for regulating pathogenic hantavirus disease. Here we propose to define the viral protein components that are responsible for attaching the virus to the 23 PSI domain. Small pieces of hantavirus surface proteins will be analyzed for their ability to bind 1v23 PSI domains, block hantavirus infection, elicit antibodies that bind hantaviruses and prevent hantavirus interactions with cellular 1v23 receptors.